This invention relates generally to radiant beam focusing arrangements and, more particularly, to an optical transducer, and a focusing system that employs it, for positioning a focusing lens so as to maintain a radiant beam accurately focused at a desired moving surface.
In the prior art it has been known to use optical focusing systems for maneuvering a lens in a light beam path in response to signals detected by a photosensitive detector to position the lens for optimum focus. For example, one type of video disc player utilizes a disc rotated on a turntable, wherein the video disc is scanned by a light beam which is accurately focused by an objective lens onto the information tracks of the video disc surface. A reflected beam of light is returned from the information track through the objective lens and directed to a photocelltype detector system, where a plurality of detectors derive both control signals for controlling, among other things, the position of the lens and an information signal containing the video information stored in the information track. The information signal output from the detector system is used to create a display, for example on a cathode ray tube to provide a video screen output.
Typically, the light beam is focused on a micron wide information track of the video disc. This requires a capability for accurately focusing the light beam to a spot of about one micron in width on the video disc. It will be readily appreciated that slight variations in the distance between the objective lens and the video disc information track can cause a loss of information in the reflected beam and undesirable frequencies can be introduced in the resulting electrical signal.
Video discs are subject to surface imperfections which vary the distance between the objective lens that focuses the impinging light beam and the information track. Video discs made of plastic, for example, may have varying thicknesses which are unavoidable even in the most sophisticated of replication processes. In disc manufacturing, further, ripple patterns may develop and vertical eccentricities can result from mass manufacturing techniques. Warping from distribution, storage and manufacturing may vary the lens-to-surface distance, especially at the outer edges of the disc where the variations move passed the light beam at a more rapid rate.
Such variations, if known beforehand, can be accommodated by several techniques. The light source, for example, theoretically can be manipulated to re-establish focus, or the beam intermediate to the light source and the objective lens also can be manipulated, although neither of these approaches are very practical. The objective lens itself may be moved toward and away from the video disc along the path of the light. Such movement of the objective lens, however, requires precision apparatus sensing very miniscule spatial movements of the video disc. The lens servo apparatus, moreover, must be capable of determining the correct direction along the reflected light beam axis to insure correct spot focusing on the disc.
Servo-systems utilizing photocell-type detector systems for focusing have been known in the past. Such a system can be seen in the Application of James E. Elliott, Ser. No. 454,130 filed Mar. 25, 1974 and now U.S. Pat. No. 3,997,715, and assigned to the assignee of the present invention, and in the patent to Kramer, et al., U.S. Pat. No. 3,876,841. In these focusing systems, photosensitive focus detectors, separate from an information detector, are positioned in the path of the light beam or adjacent the information detector. Thus, focus control signals separate from the information signal are derived for operating a servo-system for re-establishing the precise focus. One drawback in these systems, however, is that light is prevented from reaching the information detector by the focus detectors in order to re-establish the correct focus.
The systems of the known prior art have met with varying degrees of success. Many systems depend upon mechanical moving parts between the objective lens and the information detector for providing differential signal detections capable of initiating servo machanisms. Any such mechanical, interconnected device necessarily introduces delays which may materially affect the response time and other desired efficiencies in the movement of whatever element which will re-establish the desired focus. Also, many such focusing systems are very sensitive to tracking errors.
It has long been sought to have an optical focusing system for focusing a beam at a moving surface having slight variations therein, the system having minimal mechanically interconnected devices and no depletion of the reflected beam by focus detectors, which can result in loss of information. It is also desired to have an optical focusing systems having detectors of minimum size for packaging considerations, while retaining sufficient size for accurate readings. It is also desired to have a non-moving detector arrangement capable of microelectronic production in an optical focusing system arrangement.
It has also been a goal to have an improved optical transducer in a single integral unit which can provide all the necessary electrical signals from which the information content of the impinging light beam, as well as the information necessary to correct focus, can be obtained. Prior art systems have utilized detector arrays including several individual detectors arranged in predetermined patterns which are interconnected in various configurations to provide the necessary information.